1. Field of the Invention
The present invention generally relates to fluorescent lamps and, more particularly, to fluorescent lamps which have coating layers compatible with a recycling method.
2. Discussion of the Art
Fluorescent lamps typically contain a phosphor layer, containing rare-earth phosphors, and a base, or barrier, layer. The base layer, typically made of alumina or halophosphor, is applied between the phosphor layer and a glass tube. Presently, there are two types of recycling processes used to reclaim the materials contained in the phosphor layer and the base layer. The first process, also referred to as crush and sift, involves crushing a fluorescent lamp into relatively small pieces and separating the pieces by sifting. The materials contained in the phosphor layer and the base layer can be collected in powder form. The second process involves removing the ends of the lamp by cutting. Then the phosphor layer and the base layer are simultaneously removed by directing a stream of air through the tube. The components of the phosphor layer and the base layer are blown out of the tube in powder form and are collected in a collection chamber. In either method, the rare-earth phosphors and the base layer components are combined. The base layer components then need to be removed from the rare earth phosphor components in order to reuse the rare earth phosphor components in lamp products with an acceptable light output performance. Separation of halophosphor, alumina and rare earth phosphors is difficult due to the similar physical properties and particle size of the materials. No readily available methods are known to sufficiently separate the rare earth phosphors from the other materials. Therefore, there is a need in the art for a fluorescent lamp from which the rare earth phosphors of the phosphor layer may be independently recovered.
It is therefore seen to be desirable to provide a process for recovering rare-earth phosphors from the interior of a lamp envelope. The interior of the envelope defines a wall and a base layer is adhered to the wall. A coating layer containing the rare-earth phosphors is adhered to the base layer. The process includes flowing a gas through the envelope at a rate sufficient to remove particles of the coating layer but not the base layer. The process also includes collecting the particles of the coating layer, the particles containing the rare-earth phosphors.
In accordance with one embodiment of the present invention, an envelope for a discharge lamp includes a light-transmissive substrate having an inner surface. A base layer is coated on the inner surface of the envelope, the base layer comprising a blend of alumina particles, the alumina blend having 5 to 80 weight percent gamma alumina with a median particle size in the range of 30 to 100 nm and 20 to 95 weight percent alpha alumina with a median particle size in the range of 500 to 1500 nm. The base layer is applied in a suspension, the suspension containing the alumina particles and surfacting, dispersing and thickening agents, the suspension being 0.01 to 5 weight percent surfacting agent. A phosphor layer is coated on an inside surface of the base layer, the phosphor layer comprising a rare-earth phosphor blend.